The present invention is directed to a method and apparatus for desorbing water molecules adsorbed in the inner-wall surfaces of a vacuum chamber, to which a vacuum pump or pumps are connected in order to establish a vacuum therein. In order to establish a vacuum within a chamber, it is necessary to remove all gases contained in the chamber such as air and water molecules. The reason for the need to remove such gases is to reduce any partial pressures contributed by these extraneous gases. The removal of the air is quite simple, this being achieved by the action of the pump itself. The removal of the water molecules, however, is not so simple. Since water molecules are polar, there is a distinct distribution of charge within each molecule. Owing to this, there is an attraction between the ions of the chamber material, and the opposite charge associated with the polar molecule. A weak bond is thus formed, thus holding the water molecule to the surface of the material, which later on may be separated from the chamber wall to thus contribute to a partial pressure within the chamber. For this reason, it is advantageous to remove as much of the adsorbed water molecules from the interior of the vacuum chamber, to thus prevent any later contribution to partial pressure in the chamber. Techniques have been known by which the water molecules are given enough energy to break the weak bond binding it to the inner surface of the chamber, thereby breaking free from the inner surface, to thus be sucked away by the action of the pump or pumps associated with the vacuum system. Such prior art techniques have used sonar energy, by which ultrasonic waves have been directed to the outer, exterior surface of the vacuum chamber wall, by which the water molecules on the inner surface wall are excited and, thereafter, broken free from the chamber wall and eventually sucked away by the action of the pump. This is a time consuming process, and one that is not entirely successful in removing a desirable amount of the adsorbed water molecules.
Another method that has been employed to a greater degree of success, has been the use of heating the exterior wall surface of the vacuum chamber, which, by conduction, reaches the inner surface wall of the chamber, thereby thermally exciting the water molecules to thereby break the bond holding it to the chamber wall. Infrared radiation is one form that has been used for such thermal heating. However, this is also a very time-consuming process, one that requires specially-placed flanges and joints for connecting the equipment to the chamber, and special material that will withstand the high temperatures needed to create the energy necessary to break the weak bonds of the molecules. Further, this is a time consuming process, one which also does not lead to the general removal of all of the water molecules from the inner surface. The temperatures typically needed in this "bake-out" process may reach up to 450.degree. C. Such high temperatures require metal gaskets that will withstand temperatures that would otherwise break down rubber materials, or at least cause them to vulcanize. There are rubber materials extant that can exist at such high temperatures, but they do not always share properties that make them easy to use with commercial vacuum-sealing techniques. When using the metal gaskets for the thermal bake-out process, it has been usual to use copper gaskets. However, these suffer from some drawbacks. They can only be used once, they require a great many flange-bolts with high bolting torque, and in general require too much work and are too expensive for most commerical processes.
There are other non-thermal processes by which water desorbtion may take place. One such non-thermal technique is the use of a bled-in gas, such as nitrogen, which is sucked into a partially-evacuated chamber during pump down. This bled-in gas transfers its energy to the water molecules on the inner surface of the vacuum chamber, which energy is achieved by the expansion of the gas upon its entry into the partial vacuum. Thus, the desorbed water molecules are carried away through the pumping system along with the bled-in gas. This system, in the process of desorbing the water molecules, has not met with much commercial success and use, because of the additional expense required for using an exterior gas such as nitrogen. Further, the amount of bled-in gas needed for desorbing the water molecules cannot usually be predetermined, and, even with the use of a large quantity of such bled-in gas, the results are random and unpredictable, since the partial vacuum of the chamber contributes to the energy imparted to the accelerated gas, such partial vacuum needed for a better performance not a priori being known. Further, the collisions of the nitrogen molecules are random, as is well known, thus meaning that there is a very good likelihood that some inner surface areas of the vacuum chamber would not be bombarded with deflected nitrogen molecules.
Another non-thermal technique that is known utilizes a de-focused electron beam generated within the vacuum chamber. As in the case with the bled-in nitrogen gas, the de-focused electron beam impacts against the adsorbed water molecules on the inner surface walls of the vacuum chamber, exciting them sufficiently to cause desorbtion. However, this technique has, to all intents and purposes, not been utilized commercially at all.
Hitherto, all of the prior art techniques above-described have used either the impartation of sufficient energy to the water molecules adsorbed on the inner surface of the vacuum chamber either by mechanical transference, as in the case of the use of the ultrasonic wave technique, or thermal energy, as in the case of bake-out and infrared radiation. The use of the electron beam would also fall within the category of the impartation of energy to the water molecules via mechanical excitation. All of these above-described prior art techniques are not only time consuming and less than successful in eliminating partial pressure within the vacuum chamber, but have proven to be, to one degree or another, less than satisfactory in commercial uses.
It would, therefore, be highly advantageous to develop a new process for the desorbtion of water molecules adsorbed to the inner wall surface of a vacuum chamber that would be quicker in its performance, cause a greater amount of desorbtion as compared with prior art techniques, and be relatively inexpensive as compared to currently-used prior art techniques. This would not only save in costs of achieving such desorbtion, but would, in the end, allow for even a greater degree of vacuum-attainment. Further, associated herewith, it would be advantageous, in combination with a reduction of cost in desorbing the water molecules from the vacuum chamber, to do so in a much more simple and easier way that would not require the inclusion of special gaskets, O-rings and the like, nor the special equipment associated with prior art techniques. If a new technique could be found that could do away with the expensive and/or technically-advanced equipment previously used for desorbtion of water molecules, not only cost savings, but the skill of the labor performing the process need not be at as high a level. Thus, a new technique by which the desorbtion of the water molecules from the interior surfaces of the vacuum chamber is achieved by relatively simple, inexpensive, and unskilled labor would prove to be highly advantageous and cost-effective.
It is, therefore, the main objective of the present invention to provide a novel method by which the desorbtion of water molecules can be achieved in a relatively simple manner utilizing standard and conventional hardware. The method of the present invention utilizes not thermal excitation or mechanical excitation, but electromagnetic excitation in the ultraviolet range.